rhinrichs97

12-20-2000, 07:39 AM

OK. I have been reading all of this with some fascination. While the

computational aspects are fine and dandy, I think an important fundamental

problem comes down to how we teach it to our undergraduate (and graduate)

students, thereby reflecting how we think of it ourselves. Here is my

$0.02:

We need to be consistent in the way we teach students (and the general

public, for that matter). IF we use centrifugal force to explain what

happens to us when we sit in an automobile as it rounds a corner (and

noting that objects tend to slide laterally along the bench seat), then we

are also obligated to explain the similar phenomenon of what happens to our

bodies when we slam on the brakes of that same car and objects fly toward

the windshield. Centrifugal force (or what is sometimes called "reversed

inertial force") is used a lot to explain what happens in the former

(rotational) case by the same people who use Newton's first law to explain

what happens in the latter (translational) case. Those people should also

use the concept of "reversed inertial force" to explain what makes those

objects fly forward into the windshield of the car. Or, alternatively,

those people should use Newton's first law to explain what happens when you

drive around a corner in your car. You can't have it both ways. Either use

inertial reference frames for everything, or introduce the concept of

non-inertial reference frames and apply them to everything. Don't teach the

students two different ways. I explain each situation both ways to the

students and ask them which way is easist to understand. They don't like

the idea of using an accelerated reference frame in the latter

(translational) case. They really like using Newton's first law and the

concept of inertia making the objects tend to move forward at the same speed

the car was moving before it slowed down. Then, I say, "OK, you have to use

the same logic to explain what happens when the car moves around the

corner." Yet, because the concept of centrifugal force is so ingrained in

the general population, students come in with this and can't give it up.

They want it both ways. Yet, we should not teach them in an inconsistent

manner. So much for my $0.02.

--Rick

Richard N. Hinrichs, Ph.D.

Dept. of Exercise Science and Physical Education

Arizona State University

Box 870404

Tempe, AZ 85287-0404

(1) 480-965-1624

(1) 480-965-8108 (fax)

hinrichs@asu.edu (email)

www.asu.edu/clas/espe (Dept. web page)

-----Original Message-----

From: Ton van den Bogert [mailto:bogert@BME.RI.CCF.ORG]

Sent: Tuesday, December 19, 2000 8:24 AM

To: BIOMCH-L@NIC.SURFNET.NL

Subject: Re: Centrifugal Force

"Dr. Chris Kirtley" wrote:

> As far as I know, we have no sensors for segment acceleration - only

> (conceivably) joint angular acceleration, via spindles, joint afferents

> and skin receptors. Would this variable be sufficient, I wonder, for the

> CNS to compute the inverse dynamics?

In principle, yes, I think. With eyes closed, we have no information

about our motion relative to an inertial reference frame. But we

have a set of "accelerometers" in one of our rigid body segments,

the vestibular system in the head. Then we have sensory information about

relative motion of all our other body segments relative to the head.

If the CNS wanted to compute inverse dynamics in a reference frame

attached to the head, it could, theoretically. It is another question

whether this is possible in practice, considering the errors in the

sensory signals and errors introduced by the computation in neural

circuits.

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD

Department of Biomedical Engineering

Cleveland Clinic Foundation

9500 Euclid Avenue (ND-20)

Cleveland, OH 44195, USA

Phone/Fax: (216) 444-5566/9198

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computational aspects are fine and dandy, I think an important fundamental

problem comes down to how we teach it to our undergraduate (and graduate)

students, thereby reflecting how we think of it ourselves. Here is my

$0.02:

We need to be consistent in the way we teach students (and the general

public, for that matter). IF we use centrifugal force to explain what

happens to us when we sit in an automobile as it rounds a corner (and

noting that objects tend to slide laterally along the bench seat), then we

are also obligated to explain the similar phenomenon of what happens to our

bodies when we slam on the brakes of that same car and objects fly toward

the windshield. Centrifugal force (or what is sometimes called "reversed

inertial force") is used a lot to explain what happens in the former

(rotational) case by the same people who use Newton's first law to explain

what happens in the latter (translational) case. Those people should also

use the concept of "reversed inertial force" to explain what makes those

objects fly forward into the windshield of the car. Or, alternatively,

those people should use Newton's first law to explain what happens when you

drive around a corner in your car. You can't have it both ways. Either use

inertial reference frames for everything, or introduce the concept of

non-inertial reference frames and apply them to everything. Don't teach the

students two different ways. I explain each situation both ways to the

students and ask them which way is easist to understand. They don't like

the idea of using an accelerated reference frame in the latter

(translational) case. They really like using Newton's first law and the

concept of inertia making the objects tend to move forward at the same speed

the car was moving before it slowed down. Then, I say, "OK, you have to use

the same logic to explain what happens when the car moves around the

corner." Yet, because the concept of centrifugal force is so ingrained in

the general population, students come in with this and can't give it up.

They want it both ways. Yet, we should not teach them in an inconsistent

manner. So much for my $0.02.

--Rick

Richard N. Hinrichs, Ph.D.

Dept. of Exercise Science and Physical Education

Arizona State University

Box 870404

Tempe, AZ 85287-0404

(1) 480-965-1624

(1) 480-965-8108 (fax)

hinrichs@asu.edu (email)

www.asu.edu/clas/espe (Dept. web page)

-----Original Message-----

From: Ton van den Bogert [mailto:bogert@BME.RI.CCF.ORG]

Sent: Tuesday, December 19, 2000 8:24 AM

To: BIOMCH-L@NIC.SURFNET.NL

Subject: Re: Centrifugal Force

"Dr. Chris Kirtley" wrote:

> As far as I know, we have no sensors for segment acceleration - only

> (conceivably) joint angular acceleration, via spindles, joint afferents

> and skin receptors. Would this variable be sufficient, I wonder, for the

> CNS to compute the inverse dynamics?

In principle, yes, I think. With eyes closed, we have no information

about our motion relative to an inertial reference frame. But we

have a set of "accelerometers" in one of our rigid body segments,

the vestibular system in the head. Then we have sensory information about

relative motion of all our other body segments relative to the head.

If the CNS wanted to compute inverse dynamics in a reference frame

attached to the head, it could, theoretically. It is another question

whether this is possible in practice, considering the errors in the

sensory signals and errors introduced by the computation in neural

circuits.

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD

Department of Biomedical Engineering

Cleveland Clinic Foundation

9500 Euclid Avenue (ND-20)

Cleveland, OH 44195, USA

Phone/Fax: (216) 444-5566/9198

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For information and archives: http://isb.ri.ccf.org/biomch-l

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